1. Field of the Invention
The present invention relates to a hydrocarbon adsorbent state determining apparatus for determining the state of an adsorbent, including a deterioration thereof, arranged in an exhaust system of an internal combustion engine for adsorbing hydrocarbons and moisture within exhaust gases.
2. Description of the Prior Art
Some internal combustion engines are provided with an adsorbent for adsorbing hydrocarbons in exhaust gases upon starting. The adsorbent has, for example, zeolite on the surface such that hydrocarbons in exhaust gases introduce into pores of zeolite when they pass through the adsorbent, and are adsorbed by the adsorbent. The adsorbent desorbs the hydrocarbons once adsorbed thereby when the adsorbent is heated by exhaust gases to a predetermined temperature or higher (for example, 100-250° C.), permitting the desorbed hydrocarbons to be recirculated to the internal combustion engine through an EGR pipe and the like and processed without being emitted to the atmosphere. While the adsorbent repeats the adsorption and desorption of hydrocarbons as described above, the remaining amount of hydrocarbons not desorbed may gradually increase in the adsorbent, or pores of the adsorbent may be broken during a long-term use. As a result, the adsorbent is deteriorated, causing a gradual degradation in the ability of the adsorbent to adsorb the hydrocarbons. When the internal combustion engine is started with such a deteriorated adsorbent, hydrocarbons not adsorbed by the adsorbent are emitted to the outside. It is therefore necessary to determine the state, more particularly, a deterioration of the adsorbent.
The applicant has already proposed an adsorbent state determining apparatus for determining a deterioration of an adsorbent as described above, for example, in Japanese Patent Application No. 2001-323811. This state determining apparatus determines a degradation of the adsorbent in the ability to adsorb hydrocarbons, i.e., a deterioration of the adsorbent by detecting the humidity of exhaust gases using a humidity sensor, with attention focused on the fact that the adsorbent has the abilities to adsorb hydrocarbons and moisture which are highly correlated to each other, and that the humidity of exhaust gases passing through the adsorbent reflects the ability of the adsorbent to adsorb moisture. Specifically, the state determining apparatus measures a time actually taken by a detected humidity of exhaust gases, which have passed through the adsorbent, by a predetermined value associated with the adsorbent tending to be saturated with moisture within the exhaust gases adsorbed thereby after the start of the engine. Then, the state determining apparatus determines that the adsorbent is deteriorated when the measured time is shorter than a predetermined time, from the fact that the detected humidity rises at a high rate, indicating that the moisture adsorbing ability is degraded.
As described above, the state determining apparatus employs the time taken by the humidity of exhaust gases which have passed through the adsorbent after the start of the internal combustion engine by the predetermined value as a parameter representative of the ability of the adsorbent to adsorb moisture to determine the state of the adsorbent including a deterioration. However, the ability of the adsorbent to adsorb moisture varies depending on the flow velocity of exhaust gases which can fluctuate depending on an operating condition of the internal combustion engine and the like. For example, the ability of the adsorbent to adsorb moisture is degraded as exhaust gases flow at a higher velocity. Presumably, this is because exhaust gases come in contact with the adsorbent with a lower probability for a shorter time as the exhaust gases flow at a higher velocity (more likely to pass through the adsorbent), resulting in a lower apparent ability of the adsorbent to adsorb moisture. For this reason, when exhaust gases flow at a high velocity, the state determining apparatus can erroneously determine, from the under-evaluated ability of the adsorbent to adsorb moisture, that the adsorbent is deteriorated though it is in fact normal. On the other hand, when exhaust gases flow at a low velocity, the state determining apparatus can erroneously determine, from the over-evaluated ability of the adsorbent to adsorb moisture, that the adsorbent is normal though it is in fact deteriorated. Thus, the state determining apparatus can fail to ensure a sufficient accuracy for determining a deterioration of the adsorbent, and therefore still leaves room for improvement in this respect.